An analog oscillator circuit, such as a Voltage-Controlled Oscillator (VCO) receives an analog voltage signal as input to control the oscillation frequency of the oscillator. The voltage range of the control voltage received by the oscillator is limited and in recent years the voltage range of the control voltage has been reduced as circuit fabrication processes have achieved shorter channel lengths in the transistors forming some or all of the analog oscillator circuit. With such a limited control-voltage range and in order to accommodate a wide frequency range of the output of the oscillator, the oscillator is often configured to have a high frequency gain. However, such oscillators produce noisy or jittery output signals due to minor voltage shifts (i.e., noise) in the control voltage produces a change in the frequency of the output proportional to the frequency gain of the oscillator. That is, the oscillator with a large frequency gain generates larger frequency shifts for a given voltage disturbance or noise at the control voltage.
For those applications where the jitter characteristics of a high-gain oscillator are not acceptable, a low-jitter oscillator is needed such as, an LC (inductor-capacitor) voltage-controlled oscillator where the frequency is determined by the inductor and the capacitor of the circuit. However, the frequency gain or frequency range or frequency tunability of an LC VCO is limited (i.e., +/−3% over the control voltage range). Consequently, should the value of the inductance or the capacitance deviate from the expected value during the VCO manufacturing process (i.e., process variations), or deviate due to temperature variations or deviated due to supply voltage variations the oscillator will not be capable to reach the desired operation frequency because of the limited range of the control voltage acting upon the low frequency gain of the oscillator. In other words, the range of the control voltage is not sufficient to tune the oscillator to the desired frequency.
A high-frequency-gain VCO has sufficient frequency range to synthesize a desired frequency range and to tolerate variations in process parameters, temperature and voltage supply. However, the high output jitter characteristic (due to its high frequency gain among other things) of such a VCO makes it unusable for some applications. On the other hand, an LC VCO suitable for low-jitter applications has limited frequency range therefore requiring, once fabricated, calibration to compensate for the changes in VCO frequency due to process variations. However, the calibration or tuning of an analog oscillator once fabricated is difficult, inefficient, and time consuming.